1. Field of Invention
This invention concerns a process for forming a coating on a substrate by condensation of a coating material onto the substrate while the substrate is moving through an enclosure under vacuum in which evaporation of the coating material takes place.
2. Description of the Related Art
Evaporation processes are known to produce high deposition rate. Unfortunately evaporation exhibits in some cases adhesion, structure and homogeneity problems (especially for relatively considerable coating thickness).
Evaporation specialists know that the way to improve adhesion and structure is to heat the substrate.
Sputtering process is another way to coat substrates. This technique is well known to produce better adhesion, structure and homogeneity than pure evaporation has the drawback to have low deposition rate.
A low deposition rate must be avoided for most industrial use. For that reason evaporation on a heated substrate is the standard PVD process for xe2x80x9cthickxe2x80x9d coatings.
Each kind of substrate has its own upper process temperature in relation with its physical properties. Some substrates such as, for example, plastics, paper, and quenched steel, have very low process temperature and, as such, any improvement of adhesion by heating is not possible.
Krutenat described a process for the formation on a stationary substrate of a coating which includes in combination the technique of evaporation and condensation under vacuum with the technique of cathodic sputtering. It has been observed that by applying these two techniques simultaneously a mutual synergistic effect is produced and has the possible result of reducing the drawbacks exhibited by each of these techniques when used separately. Nevertheless, the Krutenat process still requires that the substrate be heated to ensure good structure and adhesion at any deposition rate.
A way to improve adhesion and structure without special substrate heating or special reduction of deposition rate.
A way to control and to maintain constant adhesion and structure even if high coating rate must be reached or if deposition rate must be modulated for external reason (for example: continuous coating line producing with the same tools different coating thicknesses at variable line speeds).
Under this invention, usage is made, within an enclosure, of a target exhibiting a superficial layer directed towards the substrate. The superficial layer contains a coating material precursor (or element). The coating is formed by evaporation followed by condensation while simultaneously cathodic sputtering the precursor from the target on the substrate.
According to this invention, adhesion and structure is maintained constant by controlling the ratio between the quantities of the precursor that are simultaneously evaporated and sputtered.
Because evaporation rate is physically controlled by target temperature and because sputtering rate is controlled by electrical discharge power density, this invention propose to regulate (with the lowest delay time) target temperature and plasma power density altogether in relation with aimed deposition rate and aimed structure and adhesion.